1. Field of the Invention
The present invention relates to an energy storing apparatus and, more particularly, to an energy storing apparatus for storing electric energy with a combination of a lead-acid storage battery and a flowing type battery.
2. Description of Related Prior Art
The proportion of total energy consumption accounted for by electric power has been increasing year by year, because the electric power can be easily converted into various forms of energy and can be easily controlled, while environmental pollution is not caused when electric power is consumed. A unique feature of electric power supply is that the production and consumption of electric energy are performed simultaneously. Under this technical limitation, it has been a major concern of those in the field of power technology to ensure that electric power of constant frequency, constant voltage, and high quality can be supplied with high reliability in conformity with variations in the total amount of electric power consumption. In the present situation, nuclear power generators or high efficiency coal/oil fired power generators are operated at a rated operating point which gives maximum efficiency at as high a level as possible, but though their generating efficiency is high, it is difficult to vary their output level. Thus hydraulic power generators or the like which are suitable for varying the amount of electric power supplied in conformity with variations in the amount of electric power consumption are also operated, thereby coping with a large increase in the demand for electric power in the daytime. The surplus electric power made available during the night by the more economical nuclear power plants or high efficiency coal/oil fired power generators is stored by way of pumped storage power generation. However, as the conditions for location of the plants for pumped storage power generation gradually become more restrictive, the power storing system which utilizes secondary batteries has been taken up.
On the other hand, it is considered that in addition to the conventional kinds of hydraulic power generation, thermal power generation and nuclear power generation, the amount of electric power supplied by solar photovoltaic power generation, wind power generation, or the like will also increase in the future. However, the amount of power generated by solar photovoltaic power generation, wind power generation, or the like is influenced by such factors as the sunshine and wind conditions prevailing. Therefore, at least on the ground, such types of solar photovoltaic and wind power generation cannot on their own become a sufficient source of power supply as a stable power supply source will only become available when they are combined with some other type of energy storing equipment.
Under the necessity, as mentioned above, for secondary batteries, many lead-acid storage batteries are used in association with solar photovoltaic power generation or the like. One other useful type of secondary battery which has been receiving attention is the battery which utilizes electrolyte flowing cells, i.e., the flowing type battery.
Kaneko et al's U.S. Pat. No. 4,362,791 patented on Dec. 7, 1982 and entitled "Redox Battery" discloses an outline of the principle of the redox flow battery as an example of the flowing type battery. This type of battery will now be described with reference to FIGS. 1 and 2.
FIG. 1 shows the state subsisting when an energy storage system using the redox flow battery is being charged.
FIG. 2 shows the state subsisting when the system is discharged.
In these figures, reference numeral 1 denotes a power generating station; 2 a substation; 3 a load; 4 an inverter/converter; and 5 a redox battery. The redox battery 5 comprises: tanks 6a, 6b 7a and 7b; pumps 8 and 9; and a flow cell 10. The flow cell 10 includes a positive electrode 11, a negative electrode 12, and a diaphragm 13 isolating electrode 11 from electrode 12. Positive electrode liquid 14 and negative electrode liquid 15 are respectively contained in the left and right chambers divided by the diaphragm 13. In the example shown in FIGS. 1 and 2, the positive electrode liquid 14 is a hydrochloric acid solution containing Fe ions and the negative electrode liquid 15 is a hydrochloric acid solution containing Cr ions.
The operation will now be described.
The electric power generated by the power generating station 1 is transmitted to the substation 2 which transforms the voltage of the received electric power into the proper voltage and then supplies the power to the load 3. On the other hand, when the electric power becomes surplus during the night, the surplus electric power of AC is converted into DC power by the inverter/converter 4 and the DC power charges the redox battery 5. In this case, as shown in FIG. 1, the battery is charged while the positive and negative electrode liquids 14 and 15 are gradually transferred from the tank 6b to the tank 6a and from the tank 7a to the tank 7b by the pumps 8 and 9, respectively. Since the liquid containing Fe ions is used as the positive electrode liquid 14 and the liquid containing Cr ions is used as the negative electrode liquid 15, the reactions indicated as "Charge" in the following expressions (1) to (3) occur in the flow cell 10: ##STR1##
In this manner, the electric power is stored in the positive electrode liquid 14 and negative electrode liquid 15.
On the other hand, if the amount of power supplied is less than the amount demanded, the reactions indicated as "Discharge" in the above expressions (1) to (3) are executed, and the DC power is converted into AC power by the inverter/converter 4. The converted AC power is supplied to the load 3 through the substation 2.
Hitherto, many lead-acid storage batteries have been used for electric energy storing apparatus. However, in order to ensure that the performance of the lead-acid storage batteries is sufficient, it is necessary to perform an equalization charge, recovery charge, and the like for maintenance of the batteries. These charging operations must be performed at a voltage higher than the rated voltage of the lead-acid storage battery to be charged.
On the other hand, solar photovoltaic power generating elements are of such a nature that when they generates electric power at a voltage only slightly higher than the voltage at which power generation is performed at high efficiency, their power generating efficiency deteriorates remarkably. As a result the current value is greatly reduced or power generation cannot be performed at all. Therefore, in the case of using lead-acid storage batteries as the energy storing batteries for solar photovoltaic power generation, the rated voltage of the lead-acid storage batteries is set in accordance with the voltage for efficient solar photovoltaic power generation. Accordingly, unless a switching connection in the solar photovoltaic power generating panels is provided, it is impossible to perform the necessary equalization charge and recovery charge of the lead-acid storage batteries by means of the electric power produced by the solar photovoltaic power generation.
Further, if the lead-acid storage batteries are used as the energy storing apparatus, the discharge voltage decreases as the lead-acid storage batteries discharge. However, unless the particular load allows for a decrease in the discharge voltage, the end cell system is used as one method whereby any decrease in the voltage of the power to be supplied to the load may be set within an allowable range. The end cell system is a method whereby an additional battery called an end cell is connected in series with a lead-acid storage battery, thereby compensating for decreases in the discharge voltage of the lead-acid storage battery. In this case, however, another apparatus for charging the end cell is also needed.